A specific type of LED lamps uses LED filaments, i.e. strip-like LED elements which typically comprise a plurality of LEDs arranged in a row on a strip-like carrier (e.g. metal, glass, or sapphire materials). The LEDs may be coated by a coating for converting the light generated by the LEDs into a desired wavelength range. Multiple LED filaments are usually arranged inside a bulb of a lamp (e.g. a LED retrofit lamp having an Edison type lamp base, such as E27 or E14). LED filaments usually are arranged essentially parallel to the longitudinal axis of the lamp. In order to minimize the generation of shadows, the LED filaments in a lamp can be arranged with a slight deviation from said longitudinal direction, such that not all LED filaments are positioned parallel to each other.
LEDs usually show a Lambertian light distribution, i.e. the highest intensity is emitted towards a main direction (usually perpendicular to the surface of the LED) and intensity decreases in other directions with the cosine of the angle between the other direction and the main direction. Light emission in the longitudinal direction of the LED filament therefore is very limited. Accordingly, light distribution of such lamps is inhomogeneous. In particular, much less light is emitted in the longitudinal direction of the lamp than in the lateral direction. If, for example, a lamp using LED filaments is hanging from the ceiling (e.g. over a table), the area directly under the lamp receives much less light than the areas to the side of the lamp.
Such inhomogeneous light distribution often is unwanted and is for example an obstacle for obtaining Energy Star certification which requires that 90% of the measured intensity values in a vertical plane vary by no more than 25% from the average of all measured values in all planes.